! A poorly explored potential contributor to aging is the mobilization of endogenous transposable elements (TEs), which can be highly mutagenic and promote genomic instability. The goal of this proposal is to determine the functional link between activation of endogenous TEs by the transcription factor Myc and changes to lifespan and aging. Previous data showed that increasing or decreasing levels of the oncoprotein Myc in the model organism Drosophila reduced or extended lifespan, respectively. New data that forms the basis of this proposal show that Myc activates the expression of a subset of endogenous TEs. Because mobilization of TEs can cause insertional mutagenesis, genome rearrangements and DNA damage, they have been proposed to contribute to tumorigenesis and other phenotypes associated with aging. However, a significant hurdle to understanding the effects of transposon mobilization has been a lack of methods to identify de novo TE insertions, since they are unique to individual somatic cells so are extremely difficult to identify by sequencing bulk tissue DNA. Only a single cell approach such as the one described in this proposal allows the number and distribution of de novo TE insertions that occur with age in somatic cells to be determined. Significantly, whole genome sequencing of multiple individual indirect flight muscle (IFM) nuclei from young and old wildtype flies revealed that the number of de novo transposon insertions increased with age. The three most frequently mobilized TEs were also found to be induced by Myc. However key questions remain unanswered regarding the link between Myc levels, TE activation and aging. For example, it is not known where TEs insert within the genome during aging or the frequency at which they mobilize. Nor is it known how Myc activates the expression of a subset of TEs, or whether their activation is functionally important for aging. The central hypothesis of this proposal is that is that Myc acts as a pro-aging gene by increasing and/or altering the distribution of de novo TE insertions in somatic cells, and that this adversely affects cell and tissue function leading to decreased lifespan. This hypothesis will be tested by pursuing three specific aims: Aim 1) Compare the frequency and genomic distribution of de novo TE insertions in wildtype flies and flies with increased or decreased Myc levels. Aim 2) Define the mechanism by which Myc activates the expression of specific TEs. Aim 3) Determine the functional link between Myc levels, TE activation and changes to lifespan. These analyses are significant because the activation of TEs by Myc could contribute to a range of age-related phenotypes, and suppressing their mobilization may provide a new therapeutic avenue to improve healthspan. The approach is methodologically innovative because it uses novel single cell genomic analyses to identify low abundance TE insertions that cannot be detected using whole tissue or organism approaches. It is conceptually innovative because interventions are tested that will for the first time define the functional contribution of de novo TE insertions to aging.